Bottom Line:
Supercapacitor technology is an extremely timely area of research with fierce international competition to develop cost-effective, environmentally friendlier EC electrode materials that have real world application.Interestingly, when Li2SO4 is used, the voltage window is extended up to 1.7 V (in contrast to 1.1 V in H2SO4).Thus, the amount of energy stored is increased by 50% compared to H2SO4 electrolyte, enabling this environmentally sound Li2SO4-based supercapacitor to deliver ~12 Wh kg(-1) at a high power density of ~2 kW kg(-1).

ABSTRACTSupercapacitor technology is an extremely timely area of research with fierce international competition to develop cost-effective, environmentally friendlier EC electrode materials that have real world application. Herein, nitrogen-doped carbons with large specific surface area, optimized micropore structure and surface chemistry have been prepared by means of an environmentally sound hydrothermal carbonization process using defatted soybean (i.e., Soybean meal), a widely available and cost-effective protein-rich biomass, as precursor followed by a chemical activation step. When tested as supercapacitor electrodes in aqueous electrolytes (i.e. H2SO4 and Li2SO4), they demonstrate excellent capacitive performance and robustness, with high values of specific capacitance in both gravimetric (250-260 and 176 F g(-1) in H2SO4 and Li2SO4 respectively) and volumetric (150-210 and 102 F cm(-3) in H2SO4 and Li2SO4 respectively) units, and remarkable rate capability (>60% capacitance retention at 20 A g(-1) in both media). Interestingly, when Li2SO4 is used, the voltage window is extended up to 1.7 V (in contrast to 1.1 V in H2SO4). Thus, the amount of energy stored is increased by 50% compared to H2SO4 electrolyte, enabling this environmentally sound Li2SO4-based supercapacitor to deliver ~12 Wh kg(-1) at a high power density of ~2 kW kg(-1).

Mentions:
Generally, porous carbons used in supercapacitors possess low packing densities, usually <0.5 g cm−342. This means that, in spite of their high specific gravimetric capacitance, many of these materials undergo a significant reduction in volumetric performance that makes them less competitive for portable, compact energy storage systems434445. However, the density of the electrodes fabricated with the microporous carbons developed in this work is relatively high, in the 0.58 to 0.85 g cm−3 range, which leads to high volumetric capacitances in the 150–210 F cm−3 range at 0.2 A g−1 (see Table 2), values which compare very well with those of other materials reported in the literature (see Supplementary Fig. S7 online). In addition, these activated carbons show relatively high rate capabilities, as can be deduced from Fig. 6. Thus, the samples obtained at low activation temperatures (i.e. AS-600 and AS-650) can withstand current densities of up to 20 A g−1 with ~50% of capacitance fading. More importantly, the AS-700 and AS-800 samples are able to maintain good capacitance values of ca. 140 F g−1 (87 F cm−3) and 170 F g−1 (99 F cm−3) respectively at a high current density of 40 A g−1, as well as remarkable capacitance retentions of 45 and 56% respectively for a 400-fold discharge rate increase from 0.2 to 80 A g−1. Thus, if both gravimetric and volumetric performances are taken into consideration, the carbon synthesized at the lowest temperature (i.e. AS-600) would appear the most appropriate as a supercapacitor electrode for low to moderate discharge rates (<5 A g−1), whereas the highest temperature carbon AS-800 would be more suitable for high rates. It is clear from Supplementary Table S2 online that the activated carbons developed in this work compare favorably with high-performing EC materials reported so far in the literature, including graphene/graphene-like materials or advanced activated carbons.

Mentions:
Generally, porous carbons used in supercapacitors possess low packing densities, usually <0.5 g cm−342. This means that, in spite of their high specific gravimetric capacitance, many of these materials undergo a significant reduction in volumetric performance that makes them less competitive for portable, compact energy storage systems434445. However, the density of the electrodes fabricated with the microporous carbons developed in this work is relatively high, in the 0.58 to 0.85 g cm−3 range, which leads to high volumetric capacitances in the 150–210 F cm−3 range at 0.2 A g−1 (see Table 2), values which compare very well with those of other materials reported in the literature (see Supplementary Fig. S7 online). In addition, these activated carbons show relatively high rate capabilities, as can be deduced from Fig. 6. Thus, the samples obtained at low activation temperatures (i.e. AS-600 and AS-650) can withstand current densities of up to 20 A g−1 with ~50% of capacitance fading. More importantly, the AS-700 and AS-800 samples are able to maintain good capacitance values of ca. 140 F g−1 (87 F cm−3) and 170 F g−1 (99 F cm−3) respectively at a high current density of 40 A g−1, as well as remarkable capacitance retentions of 45 and 56% respectively for a 400-fold discharge rate increase from 0.2 to 80 A g−1. Thus, if both gravimetric and volumetric performances are taken into consideration, the carbon synthesized at the lowest temperature (i.e. AS-600) would appear the most appropriate as a supercapacitor electrode for low to moderate discharge rates (<5 A g−1), whereas the highest temperature carbon AS-800 would be more suitable for high rates. It is clear from Supplementary Table S2 online that the activated carbons developed in this work compare favorably with high-performing EC materials reported so far in the literature, including graphene/graphene-like materials or advanced activated carbons.

Bottom Line:
Supercapacitor technology is an extremely timely area of research with fierce international competition to develop cost-effective, environmentally friendlier EC electrode materials that have real world application.Interestingly, when Li2SO4 is used, the voltage window is extended up to 1.7 V (in contrast to 1.1 V in H2SO4).Thus, the amount of energy stored is increased by 50% compared to H2SO4 electrolyte, enabling this environmentally sound Li2SO4-based supercapacitor to deliver ~12 Wh kg(-1) at a high power density of ~2 kW kg(-1).

ABSTRACTSupercapacitor technology is an extremely timely area of research with fierce international competition to develop cost-effective, environmentally friendlier EC electrode materials that have real world application. Herein, nitrogen-doped carbons with large specific surface area, optimized micropore structure and surface chemistry have been prepared by means of an environmentally sound hydrothermal carbonization process using defatted soybean (i.e., Soybean meal), a widely available and cost-effective protein-rich biomass, as precursor followed by a chemical activation step. When tested as supercapacitor electrodes in aqueous electrolytes (i.e. H2SO4 and Li2SO4), they demonstrate excellent capacitive performance and robustness, with high values of specific capacitance in both gravimetric (250-260 and 176 F g(-1) in H2SO4 and Li2SO4 respectively) and volumetric (150-210 and 102 F cm(-3) in H2SO4 and Li2SO4 respectively) units, and remarkable rate capability (>60% capacitance retention at 20 A g(-1) in both media). Interestingly, when Li2SO4 is used, the voltage window is extended up to 1.7 V (in contrast to 1.1 V in H2SO4). Thus, the amount of energy stored is increased by 50% compared to H2SO4 electrolyte, enabling this environmentally sound Li2SO4-based supercapacitor to deliver ~12 Wh kg(-1) at a high power density of ~2 kW kg(-1).